1. Field of the Invention
The present invention relates to an image forming apparatus having a high-speed double-sided printing function, such as a copier, a printer, or a multifunction peripheral, and to a method of controlling printing therefor.
2. Description of the Related Art
Presently, image forming apparatuses with double-sided printing functions, such as a copier, a printer, or a multifunction peripheral are in widespread use. Examples of double-sided printing methods for providing a double-sided printing function include normal double-sided printing and high-speed double-sided printing.
Normal double-sided printing is a method of achieving double-sided printing on a sheet of paper by continuously performing printing on one side of one sheet of paper and printing on the other side of the same sheet of paper. For example, in normal double-sided printing, the printing is performed by first printing an image of the second page on a back surface (first printing surface) of the first sheet of paper, then printing an image of the first page on a front surface (the last printing surface) of the first sheet of paper, then printing an image of a fourth page on the back surface of a second sheet of paper, and then printing an image of a third page on the front surface of the second sheet of paper.
High-speed double-sided printing is a method of performing double-sided printing on two or more sheets of paper in parallel by printing on one surface of one sheet of paper and then printing on the other surface of the sheet of paper, between which printing on both sides of another sheet of paper is performed. For example, in high-speed double-sided printing, the double-sided printing is performed by first printing the image of the second page on the back surface of the first sheet of paper, then printing the image of the fourth page on the back surface of the second sheet of paper, then printing the image of the first page on the front surface of the first sheet of paper, then printing the image of the sixth page on the back surface of a third sheet of paper, then printing the image of the third page on the front surface of the second sheet of paper, then printing the image of the eighth page on the back surface of a fourth sheet of paper, and then printing the image of the fifth page on the front surface of the third sheet of paper.
According to the high-speed double-sided printing, the time necessary to complete the printing process can be reduced by providing an overlap between the operating period for transporting a sheet of paper to a print engine while reversing the back surface and the front surface thereof and the operating period for performing printing on the back surface or the front surface of another sheet of paper, and the double-sided printing can be performed at a higher speed than the normal double-sided printing.
When high-speed double-sided printing is performed, the sheet of paper whose back surface has been printed on is kept on a paper sheet transport path. This point can be described by taking the example of the above-described high-speed double-sided printing. In high-speed double-sided printing, while printing on the back surface of the second sheet of paper, the first sheet of paper whose back surface has been printed on is kept on the paper sheet transport path. In addition, while the printing on the front surface of the first sheet of paper and on the back surface of the third sheet of paper are being performed, the second sheet of paper whose back surface has been printed on is stored on the paper sheet transport path. In addition, while the printing on the front surface of the second sheet of paper and on the back surface of the fourth sheet of paper are being performed, the third sheet of paper whose back surface has been subjected to the printing is stored on the paper sheet transport path.
Then, immediately before the high-speed double-sided printing is finished, all of the sheets of paper stored on the paper sheet transport path are delivered, and printing is performed on the front surfaces of the sheets of paper delivered from the paper sheet transport path, completing the high-speed double-sided printing.
However, in a situation where: the first high-speed double-sided printing is performed according to one job instruction to perform double-sided printing; single-sided printing is then performed according to a job instruction to perform single-sided printing; and the second high-speed double-sided printing is then performed according to another job instruction to perform double-sided printing, the image forming apparatus finishes the first high-speed double-sided printing first before performing the single-sided printing, and performs the second high-speed double-sided printing after finishing the single-sided printing. Therefore, the printing speed is greatly reduced compared to the situation where the first high-speed double-sided printing and the second high-speed double-sided printing are continuously performed (without the intermediate step of the single-sided printing).
Further, in a situation where: the image forming apparatus continuously receives two jobs of double-sided printing; and the two jobs are different from each other due to the size or orientation of the sheets of paper specified by the jobs, the image forming apparatus does not continuously perform the high-speed double-sided printing based on one job and the high-speed double-sided printing based on the other job. This is because the method for the high-speed double-sided printing differs depending on the size or orientation of the sheets of paper.